Consolidated Balance Sheets (Unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets:
Trade accounts receivable, allowance for doubtful accounts	$ 2,286	$ 2,052
Property and equipment, accumulated depreciation	$ 120,324	$ 119,257
Stockholders' equity:
Common stock, par value (in dollars per share)	$ 0.008	$ 0.008
Common stock, shares authorized (in shares)	40,000,000	40,000,000
Common stock, shares issued (in shares)	31,252,263	31,172,666
Common stock, shares outstanding (in shares)	26,161,107	26,113,544
Treasury stock, shares of common stock (in shares)	5,091,156	5,059,122